<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        ul {
            padding: 0;
            margin: 0;
            list-style: none;
            position: relative;
        }
        .list-item {
            width: 200px;
            height: 50px;
            box-sizing: border-box;
            text-align: center;
            line-height: 50px;
            border-bottom: 1px solid #aaa;
            background-color: rgba(170, 170, 170, 0.2);
        }
        .list-item div {
            width: 150px;
            height: 150px;
            background-color: rgba(170, 170, 170, 0.8);
            position: absolute;
            left: 200px;
            top: 0;
            display: none;
        }
        .list-item:hover{
            background-color: rgba(170, 170, 170, 0.8);
        }
        .list-item:hover div{
            display: block;
        }
    </style>
</head>
<body>
    <ul>
        <li class="list-item">
            第一项
            <div>第一项内容</div>
        </li>
        <li class="list-item">
            第二项
            <div>第二项内容</div>
        </li>
        <li class="list-item">
            第三项
            <div>第三项内容</div>
        </li>
    </ul>
</body>
</html>